When asked why the Bible is not explicit about the Trinity (e.g., there being no verse that reads like "this is life eternal, to know thee, the only true God: Father, Son, and Spirit"), Matt Slick was honest (and by being honest, reveals the truly Satanical nature of Calvinism, a theological system other anti-Mormons like Bowman, White, and Durbin affirm):
Why would God not be so explicit like that? I believe it's because there are people who are not predestined to be saved and He hides the truth from those who are going to hell. ("After show of debate, Trinity vs Unity with Slick vs Xavier" [31:25 mark])
Slick then appeals to Mark 4:10-12 to support this thesis. I think Leighton Flowers did a good job answering the Calvinistic abuse of Jesus and why He taught in parables:
The Real Reason Jesus Spoke In Parables
Calvinism is a Satanical system, and it is not a fluke that really pitiful individuals (Bowman; Eklund; Anson; Durbin; Slick; Shafovaloff) hold to it—a false religious system that does not bring about regeneration.
For more, see:
An Examination and Critique of the Theological Presuppositions Underlying Reformed Theology